1. Field of the Invention
The present invention relates generally to verifying the source of digital graphic information being transmitted. More particularly, the present invention relates to compressing data and attaching a digital signature to the compressed data. The invention is useful in authenticating compressed data when lossy compression techniques are used or when transmission is interrupted.
2. Description of Related Art
In recent years, on-line service providers as well as Internet sources have provided increasingly detailed graphical images to computer users. Since these images are data intensive and are transmitted over a communication link of limited bandwidth, often standard two-pair telephone lines, various compression techniques have been used.
One of the most popular compression techniques involves breaking an image into components and transmitting the components sequentially. The components may be coefficients of a discrete cosine transform. A receiving device, usually a personal computer, receives these coefficients and uses them to progressively render the image. The more significant (or "higher order") coefficients provide the initial low-resolution image while lesser significant (or "lower order") coefficients subsequently refine the image and provide additional resolution.
During transmission of the graphical images, data loss may occur. For example, bandwidth or time limitations force termination of a transmission over the communication link. Despite the data loss resulting from a truncated transmission, a lower resolution, but usable image, may be displayed. Moreover, even when the transmission is complete, data loss may still occur if lossy compression techniques are used. "Lossy compression techniques" are compression methods which sacrifice exact replication of an image in order to achieve additional compression. Lossy compression is unacceptable for compressing certain data, such as executable computer code where even a one bit change results in nonexecutable code. However, for video data, it is sometimes worthwhile to sacrifice absolute pixel accuracy for additional compression of data. Lossy compression techniques are particularly useful in compressing an image where the additional compression is significant in reducing storage space required by the image, as well as reduced bandwidth transmissions of the image.
In casual viewing of an image, minor losses which result from lossy compression techniques are often insignificant. However, such losses can be significant for authentication techniques where it is important to verify the source of the image. Standard authentication procedures require that digital data be signed with a digital signature to verify the source of video image. However, after signing the data, any further modification, post-processing or manipulation of the data which results in data loss will invalidate the signature.
Because digital signaturing techniques are rendered useless if even one bit is changed, there exists a need for a method by which the authenticity of data can be determined, even when an incomplete data set is transmitted or when lossy compression techniques are used prior to transmission.